Cuspidine

Cuspidine is a fluorine bearing calcium silicate mineral (sorosilicate) with formula: Ca₄(Si₂O₇)(F,OH)₂.^[2] Cuspidine crystallizes in the monoclinic crystal system and occurs as acicular to spear shaped pale red to light brown crystals. It is a member of the wöhlerite group.

Cuspidine
Yellowish cuspidine crystals from the type locality at Mount Vesuvius
General
Category	Sorosilicate
Formula	Ca4(Si2O7)(F,OH)2
IMA symbol	Csp[1]
Strunz classification	9.BE.17
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 10.93 Å, b = 10.57 Å, c = 7.57 Å; β = 110.11°; Z=4
Identification
Color	Colorless, tan, light brown, pale red
Crystal habit	Minute spearhead-shaped crystals, acicular, granular
Twinning	Simple, lamellar, polysynthetic on {100}
Cleavage	Good on {001} imperfect on {110}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	5–6
Luster	Vitreous
Diaphaneity	Transparent to translucent
Specific gravity	2.85–2.96
Optical properties	Biaxial (+)
Refractive index	nα = 1.586 – 1.594 nβ = 1.589 – 1.596 nγ = 1.598 – 1.606
Birefringence	δ = 0.012
2V angle	Measured: 59° to 71°
References	[2][3][4]

Cuspidine was first described in 1876 for an occurrence in Monte Somma, Italy.^[2][4] The name is from the Greek cuspis for spear from its characteristic crystal form.^[2] Cuspidine occurs as crystals in tuff from Monte Somma. In the Franklin, New Jersey mine area it occurs in contact metamorphosed limestone. In Dupezeh Mountain, Iraq, it occurs in melilite bearing skarn. Associated minerals include augite, hornblende, diopside, grossular, biotite, phlogopite, monticellite, wollastonite, calcite, spinel, magnetite and perovskite.^[4]